Fuji 110/2 AF performance on GFX 50S

This is the 66th in a series of posts on the Fujifilm GFX-50S. The series starts here.

In this post, I reported on Fuji GFX 50S systematic autofocus errors at certain f-stops (mostly one, actually) using the 23 mm f/4 lens. I have seen similar behavior with the 63 and 120 macro. When I made the photographs for yesterday’s visual comarison of the 110 on the GFX and teh Otus 85 on the Sony a7RII, I shot some AF pix with the GFX just to see what would happen. In all cases, at least one one of my manually focussed images was sharper. At f/2.8, the manually focused ones were a lot sharper.

Thus, it was with some trepidation that I set up my controlled af test. I used the following low-contrast target:

I have marked the region of interest that I used in red. Here’s the procedure:

ISO 100

AF-S

Spot and Zone both tested (this is new; I just tested spot before)

Spot set on the ROI, which included a bit of the zone plate.

Zone set the same plate which included all the zone plate and some or all of several squares.

Release priority: focus

Pre-trigger: 500 milliseconds, which was long enough for focus confirmation green square to flash and confirmation beep to sound in all cases

Manual exposure

Wescott LED panels set to 5500 K.

Electronic shutter

Cognisys computer-driven focusing rail

32 exposures 4 mm apart

Target distance at the center of the rial, 3 m.

RAF converted to DNG in Adobe DNG Converter

DNG exported as TIFF mosaiced file in dcraw (document mode)

TIFFs cropped and raw channels selected in Matlab program

MTF50 of cropped TIFFs measured with MTF Mapper

Data assembled in Matlab

Data plotted in Excel

The spot mode data for the red and blue raw channels, and the Gr one:

What you are looking at is a standard measure of sharpness, MTF50, as measured in cycles per picture height (cy/ph) at the whole stops from f/2 through f/11. The orange line is the average, or mean, of the 32 exposures at each aperture. The gray line is that average, plus the standard deviation for the 32 exposures, and the blue line is the average minus that standard deviation. If the data were Gaussian (which it isn’t) two thirds of the values would be between the gray and the blue lines.

For comparison, here’s a run of 100 images 1.6 cm apart with the focus fixed using MF at f/2.8, the sharpest aperture for this lens, and the one that seems to give the AF algorithm the most trouble:

You can see that peak sharpness is about 1400 cy/ph. This is much lower than we get with the high contrast target, but we’re just interested in relative sharpness here. The average sharpness using AF at f/2.8 is less than that, but not by much. Focusing at f/2.8 is not very consistent, though. AF is pretty consistent at the other apertures. This is the best AF performance I’ve seen in hte four lenses tested.

Now let’s look at the results in zone Ad MODE:

At f/2.8, the AF performance is much more consistent, and the results are very close to the best you could do focusing manually.

This means that I’m going to have to go back and run the other three lenses through the same tests in zone mode.

Note that the blue channel, which was weak at f/2 in the manual focusing tests using the razor blade target, is fine here. I’m not sure what that’s about.

Good point. However, I don’t think that explains with relative differences, since blur sources tend to add as the square root of the sum of the squares. If it wre just the sharpness of the edge, I’d expect to see more difference in the channel MTF50s with the printed target.

Hi Jim,
Is there a difference in the length of the edge extracted from the low-contrast target, and the length of the edge extracted from the razor blade?

My rule of thumb is to aim for an edge length of 80 pixels. When using –bayer green, you would have to double the edge length, and in –bayer red and –bayer blue, quadruple the edge length (320 pixels!) to maintain the number of individual samples used by the slanted edge method. In practice, you can probably get away with somewhat shorter edges, but that depends on the overall SNR (across the edge).

Anyhow, a large difference in edge length could explain the observed difference in the blue channel performance.

For both the printed target and the razor blade, I have been using 200×200 pixel crops in the GFX lens tests. The exception is the 23mm lens, where I didn’t want to get close enough to get the razor blade big enough in the image for a 200×200 crop, so I used 100×100. The 200×200 crop gives 100×100 in each raw plane. The 100×100 crop gives 50×50, which is below your standards, and probably explains the noise in the resultant data.

Thanks for stopping by Frans, and for all your help. The new version of MTF Mapper is working perfectly for me.

Hi;
I am a gfx user and having focus shift and focus inconsistency problems with 120mm f4 especially at f5.6.Do you think zone focusing can solve this problem instead of using single point?Single point is always adviced for better af but i am confused.

By the way will you make a comparrison at landscape distance of 120mm and 110mm ?I really wonder if 110mm is sharper than 120mm let’s say at f4 to f11.

You are right. I had forgotten that I did that test. Note, though, that it seems to be target dependent. The f/5.6 values in the zone vs spot test weren’t as bad as in the first spot test that I did a couple of month ago.

I am speaking with Fuji lens design team in Japan about the focus shift issue.What they have told me is they measured the focus shift in different distances and fine tuned it by the last firmware.But when shared them my findings after firmware update they did not understand the problem so they started to measure the lens again.I will let you know their findings.